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Since there's no empty line between the content of mail 2 and the "From " line of mail 3, the body of mail 2 isn't really terminated by a newline. Reading the message confirms this:
>>> import mailbox
>>> mbox = mailbox.mbox('mbox')
>>> mbox[1].get_payload()
'The mail content of mail 2...'
However, according to RFC 4155 this is a bug, as each message in a mbox must be terminated by an empty line.